This patch fixes three issues that were found running
functional tests with Python 3:

1. python-requests sets 'application/x-www-form-urlencoded' as
   content-type if the input is not a string object and no
   content-type is given.

2. Encoding of the headers is now only done if required. This
   prevents comparisons between unencoded headers and encoded
   prefixes and avoids unnecessary forloop-iterations.
   One unittest was extended to ensure it works for unencoded
   and encoded headers with or without the prefix.

3. Functional tests recently switched to using byte data for
   testing, thus the comparison needs to be a byte object as well.

Change-Id: I035f8b4b9c9ccdc79820b907770a48f86d0343b4

Looks like test_shell.py got left behind when
the unit tests were relocated under tests/unit.
As a consequence they weren't included in tox
runs.

Change-Id: I93435e6141c5e3ad4a9403626f226d7c760cbd7c

Change-Id: I9580ab0f5bc53b472d0b633858090f9e0708ed90

According to http://docs.python.org/2/library/unittest.html
assert_ is a deprecated alias of assertTrue.

Change-Id: I18db0af72934dde4498502fb29579941d020cf01

Change-Id: I99a46860ffff93452904d24504a4ccc58e8e64c6